Wondering if any residential provider in the world is offering a service where you can say "sell me only renewables". There'd probably be a cost associated with this, but I think I'd have to consider it if it were available locally.
In Houston, the "Oil and Gas Capital of the World", I buy 100% wind power from Green Mountain. They're not the only game in town for 100% renewables, either. Green Mountain themselves do have mixed plans, but mine's 100% wind. It's not the cheapest power in town (some retail electric providers go below $0.04 per kWh for the first year of your contract or such if you don't care) but with Texas by law having a competitive retail electricity market there's a lot of choice.
They probably just trade it. Increase their supply so they can sell enough excess back to the grid to make up the cost later. You effectively get 100% wind, but in reality you might have got 80% wind, 20% gas and sold some wind back to the grid. It's a good interrim solution at least.
As problems stated, there's probably a bit of arbitrage going on behind the scenes. However, Texas also has a well connected grid across the rather large state that is only weakly connected to other grids. There's a lot of wind in Texas and a lot of wind power. On an average day 12% of the entire state is powered by wind. Some days it's 40%. Sometimes wind produces so much energy that the spot price goes negative and wind farms are paid to disconnect so they don't overrun the transmission system.
I think Arcadia Power is available nationally, I believe it was 1.5c/kWhr very cheap. It's the same REC offsetting that Google is doing, just at a much smaller scale.
It's so common here in NYC that there's a scam where people go door to door, trick you into believing you're signing up for 100% wind power, and then jack up prices after a few months.
Cost depends, I just used some random price search site to compare a few things, I hope that's somewhat useful data. Assuming 3500 kWh/year yearly price would be
1095 € for the local "default contract" [1]
1060 € for a one-year contract from the local default provider
1080 € for their "eco option"
792 € for the cheapest offer overall [2]
855 € for the cheapest 100 % renewable
1040 € for a reputable 100 % renewable provider that has the certification supported by most environmental organisations
[1] with no minimum contract length, generally relatively expensive since they are the "lazy" option and you can switch out at any time. Basically, if you don't have any other contract they are legally obligated to give you this one.
[2] (which is tricksy, since it is a package deal with massive overage fees, not mostly a flat fee per kWh like the others)
So there is quite a potential for savings by picking the right provider, and high-quality eco options are among the more expensive ones, but if you do research you probably can find a compromise you like. E.g. the expensive eco providers often earmark 3 % or so of the price for long-term investments in building their own projects, instead of only reselling electricity provided by others, which of course adds on top and some people want to support. Some accept biogas as a power source, others don't.
And since many people never cared and pay the default deals, when they think about switching for eco reasons the shock isn't as big, because they often can save and get something 100 % renewable. Many people are willing to pay 20-50 € extra and go for that if available. The expensive options obviously have fewer customers, but I know people from all categories.
